The chemical most often used in perming hair is called ammonium thioglycolate. It is known as a reducing agent, and when placed in an alkaline solution, it works by breaking down the disulphide bonds in the hair follicle. This softens and swells the hair's keratin, allowing the hair to be manipulated into the desired style.
Keratin is a type of protein that makes up hair. Keratin molecules in normal hair are arranged in straight bundles held together by disulphide bonds. Disulphide bonds are created by cysteine, an amino acid. The more disulphide bonds present in a single strand of hair, the straighter the hair. Ammonium thioglycolate breaks these bonds down by replacing one of the sulphur atoms within each bond. Breaking the disulphide bonds causes the keratin bundles to break apart and soften the hair. In its weakened state, the hair can be manipulated into any shape. In perming, the hair is set on perm rods, which vary in size depending on the client's curl size preferences. Once the hair has been set with curls, an oxidation solution is applied. The solution contains hydrogen peroxide, which reinforces the disulphide bonds, strengthening the hair and making the new style permanent. Because the natural keratin bonds have been broken down and reconstituted, permed hair is weaker than unprocessed hair and must be treated with care.